Air springs have been used for some time to level or to raise vehicles for whatever desired purpose, such as when the cargo changes or for off-road travel. These air springs are disposed between the wheel's axle or axle support (the unsprung mass) and the chassis (the sprung mass). When deflated, the chassis is lowered to rest upon the axle or support, the chassis being supported by, for example, rubber stops or the like. When inflated, the chassis is raised relative to the axle or support to the desired height. Shock absorbers are also typically provided between the chassis and the wheel axles or supports to dampen relative motion between the chassis and axles.
To raise the chassis, the air spring or springs are inflated, usually manually or, recently, by suitable controls. The pressure of the compressed air within the spring rises resulting in a corresponding lifting of the chassis by the air springs.
If cargo or passengers are not evenly loaded, the air springs must be adjusted to maintain the chassis at a selected height and maintain a level. It would be advantageous if the manipulation of the air springs, i.e., inflation and deflation, would occur automatically while a vehicle is traveling to thereby dynamically level the vehicle. Further, this dynamic leveling means should not require position transducers or proximity switches which, while providing a reading of the position of the chassis relative to the axle and ground, are expensive and are subject to failure.